Replication of surface deformation images

ABSTRACT

A METHOD FOR PREPARING A PLURALITY OF SUBSTANTIALLY IDENTICAL REPLICAS OF SURFACE DEFORMATION IMAGES, E.G., PHASE HOLOGRAMS IS DISCLOSED. BASICALLY, THIS PROCESS INCLUDES THE STEPS OF FORMING A FIRST GENERATION REPLICA BY CASTING AGAINST THE ORIGINAL SURFACE DEFORMATION IMAGE, FORMING A SECOND GENERATION REPLICA BY CASTING AGAINST THE FIRST GENERATION REPLICA, TRANSFERRING THE CAST SECOND GENERATION REPLICA TO A SUPPORT SURFACE, AND REPEATING THE SECOND GENERATION CASTING AND TRANSFER STEPS. THE METHOD MAY INCLUDE THE STEPS OF FORMING A METAL MASTER FROM THE SECOND OR THIRD GENERATION REPLICA AND STAMPING A PLURALITY OF SIMILAR REPLICAS USING THE METAL MASTER.

Feb. 23, 1971 w FOLGER ETAL REPLICATION OF SURFACE DEFORMATION IMAGES Filed Sept. 11. 1967 FORM SURFACE D'EFORMATION PATTERN TREAT SURFACE FORM FIRST GENERATION REPLICA BY APPLYING A HARDENABLE MATERIAL FORM SECOND GENERATION REPLICA BY APPLYING ANOTH HARDENABLE MATERIAL FORM THIRD GENERATION REPLICA BY DEPOSITING A SILVER LAYER AND A NICKEL LAYER u STAMP DEFORMABLE FORM FOURTH GENERATION MATERIAL DEPOSITING NICKEL REPLICA BY PASSIVATING AND STAM P DE FOR MABLE MATERIAL INVENTORS JOHN C.URBACH WILLIAM F. FOLGER 8% EOE-EN A TTORNEVS 3565978 QR INT-3 W;

3,565,978 REPLICATION OF SURFACE DEFORMATION IMAGES William F. Folger, Webster, and Ronald H. Cohen and John C. Urbach, Rochester, N.Y., assignors to Xerox Corporation, Rochester, N.Y., a corporation of New York 7 Filed Sept. 11, 1967, Ser.. No. 666,617 Int. Cl. B29c 1/02; B29d 11/00; G02b 5/18 US. Cl. 2641 14 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates in general to replicating systems, and more specifically, to a system for replicating surface deformation image patterns.

Two general methods are known for forming image patterns on the surface of deformable thermoplastic materials. The first of these is known as'frost imaging and is described, for example, in US. Pats. 3,196,008; 3,196,001 and 3,258,336. The other method is relief imaging and is described in U.S. Pats. 3,055,006; 3,063,- 872 and 3,113,179, for example. In each, the surface of a softened thermoplastic material responds to electrostatic forces by deforming. A fundamental distinction, however, between frost and relief is that frost occurs on uniformly charged areas as a uniform distribution of surface folds or wrinkles whereas relief responds to electrostatic gradients only, forming a single: line deformation along the edge defined by a charge gradient. Relief will not occur where there is uniform charge distribution. This is an important difference, not only from the viewpoint of application, but also because it definitely indicates a different mechanism for the two systems. In the usual frost mode of surface deformation imaging, a latent electrostatic image or charge pattern is formed on an insulating film which is softenable as by the application of heat or solvent vapor. After the latent electrostatic image is formed, the film is softened until the electrostatic forces of the charge pattern exceed the surface tension forces of the film. When this threshold condition is reached, a series of very small surface folds or wrinkles are spontaneously formed on the film surface, the depth of the wrinkles in a particular area of the film being generally dependent upon the intensity of charge in that area and the film thickness. This gives the image a frosted appearance. Alternatively, the film may be softened prior to the application of the charge pattern if the film remains sufiiciently insulating in a softened state to hold the charge. The frost image is set or fixed by allowing the film to reharden. In a reusable frost system it is usually desirable to later erase the fixed image after use by resoftening the thermoplastic film and maintaining a sufficiently low viscosity for appropriate periods of time to permit surface tension forces to smooth the film surface.

te States Patent 0 F $55,978 Patented Feb. 23, 1971 Recently, it has been discovered that surface deformation imaging is especially useful in holography. As initially described by Dennis Gabor in an article entitled, A New Microscopic Principle appearing in Nature 161, 777-778 (1948), holography is a two-step imaging process in which the defraction pattern of an object illuminated with coherent radiation such as laser light is recorded on a radiation sensitive layer. This record, known as a hologram, is then used to reconstruct an image forming wave front by reilluminating the hologram with coherent electromagnetic radiation. This process was further refined by the use of an off axis reference beam technique in which a reference beam of coherent light is brought in at an angle with respect to the beam of coherent light used to expose the hologram, thus forming an interference pattern on the photosensitive recording plate. After development, this plate or hologram is again exposed to coherent radiation and the image is reconstructed off the optical axis at an angle proportional to the angle of incidence of the original reference beam. This procedure is further described by E. Leith and J. Upatnieks, in two articles in the Journal of the Optical 'Society of America, Reconstructed Wave-Fronts in Communication Theory, 52, 1123, October, 1962 and Wave- Front Reconstruction With Continuous Tone Objects, 53, 1377, December 1963.

Holography has a great many advantages over other imaging techniques. For example, a hologram may be used to reconstruct a three dimensional image with radiation of another wavelength than that used to record the hologram, resulting in high magnification. It may be used for the secure transmission of information, since the hologram is an extremely complex coding of optical information and bears little or no resemblance to the original scene. Also, it may be used for rapidly making many similar images of the original since when the hologram is cut into a number of sections, each section contains all of the holographic information necessary to reconstruct the original image. Although most experimenters have worked With amplitude holograms, it is also possible to make phase holograms in which the image formation is stored in a phase modulating pattern rather than an amplitude modulating pattern as described, for example, in an article by G. L. Rogers in the Proceedings of the Royal Society, Edinberg, 193 (1953) and in an article by W. T. Cathey, Jr. in the'Journal of the Optical Society of America, 55, 457 (1965). Such phase holograms have previously been made from conventional silver holograms by bleaching the silver and using the phase difference introduced by gell swelling and/or refractive index changes. Although these have certain advantages over amplitude holograms, they require extremely diflicult and complex processing to produce.

Recently, it has been discovered that the above described surface deformation imaging systems may be used to produce phase holograms of excellent quality. This procedure is described in copending application Ser. No. 521,982, filed Jan. 20, 1966. Typically, according to this process phase holograms may be prepared on a deformable thermoplastic layer coated on a photoconductive layer. The deformable layer is uniformly electrostatically charged and exposed to the object and reference beams using coherent radiation to which the photoconductor is sensitive. After exposure the deformable layer is recharged and the plate is heated to the softening temperature of the deformable layer. At this temperature, spontaneous surface deformation occurs according to the holographic pattern. The layer is then cooled to fix the holographic record. If desired, the heating can take place during exposure so that the reconstructed image may be viewed simultaneously with the formation of the hologram. This process is capable of simply producing phase holograms of excellent quality and extraordinary resolution. When viewed by ordinary light, however, the surface appears to have a random pattern of very fine, almost invisible, surface ridges and valleys. Since the average depth of these ridges and valleys is often less than 1 micron, they cannot be seen with the unaided eye.

While these phase holograms have excellent qualities,

there are certain drawbacks. The materials which may be easily deformed often are low melting, have a tacky surface and are soft. Theymay be easily damaged in handling and through inadvertent contact with dust or exposure to moderately high temperatures. While the fact that the deformable plates may be erased by re'softening the layer and allowing the surface to be smoothed by viscosity forces where it is desired to reuse the plate may be desirable, this temperature sensitivity would be undesirable where a permanent record is desired. It is difficult to form replicas of an original hologram since the surface of the original is easily degraded. Casting against a holographic original from a melt is difiicult since heating the original tends to destroy the holographic pattern. Also, many of the preferred deformable materials are soluble in many of these solevnts which would be used in casting replicas from a solution. Aslo, extraordinarily high fidelity in replicating is necessary because of the fine detail included in the holographic pattern and because the amplitude differences between raised and depressed portions of the holographic pattern are often much less than 1 micron.

Thus, there is a continuing need for improved methods of preparing replicas of surface deformation originals, especially holographic originals, so as to permit the production of large numbers of replicas and to insure the permanence of the original and replica images.

It is, therefore, an object of this invention to provide a replicating system for surface deformation images which overcomes the above-noted disadvantages.

It is another object of this invention to provide a simple and economical method of replicating holographic originals.

It is another object of'this invention to provide a method of replicating surface deformation images which is capable of maintaining the highest fidelity.

It is another object of this invention to provide a method of replicating surface deformation images which is capable of reproducing exceedingly small amplitude differences.

It is still another object of this invention to provide a surface-deformation image replicating system which is capable of producing a large number of duplicate images of high fidelity and permanence.

SUMMARY OF THE INVENTION The above objects and others are accomplished in accordance with this invention, by providing a replication method which includes, generally, the steps of casting against the original surface deformation image pattern bearing surface with a curable casting material, curing the casting material to form a negative replica, casting a second generation positive replica by casting from the first generation negative replica, transferirng the cast second generation replica to a support surface, and repeating the second generation casting and transfer steps. The process may include the steps of silvering the second geneartion replica, electroforming a metal surface on the silvered second generation replica, separating the metal layer from the second generation replica to form a third generation metal negative replica and pressing a thermoplastic preform against the metal replica to form the fourth generation final positive replica of the original image or passivating the surface of the third generation metal negative replica, electroforming a metal surface on the passivated surface, separating the metal layer from the third generation negative replica to form a fourth generation metal replica and pressing a. thermoplastic preform against the fourth generation metal replica to form a final fifth generation negative replica of the final image. An-image pattern may be any surface deformation configuration which conveys intelligence to the eye or through a read-out or reconstruction system.

BRIEF DESCRIPTION OF THE DRAWING The process of this invention will be further understood upon reference to the drawing which shows a simple flow sheet pointing out this process.

As shown in the drawing, the surface deformation pattern to be replicated is first prepared by any conventional. technique. It is preferred that the surface of this original be treated to harden or toughen it, as is further described below.

A first generation replica is then prepared by casting against the original surface with a material which does not adversely affect the original.

A second generation replica is then cast from the first generation replica.

The thus formed second generation replica is then transferred to a support surface to form a final positive replica of the original image and the first generation replica is reemployedas a master or thin silver layer is formed on the'second generation replica and a thicker nickel layer is electroformed thereon.

If a silver and nickel third generation replica is formed, it may be used either as a short run stamper, or a tougher fourth generation nickel master may be formed thereagainst after passivation for use as a long run stamper.

Of course, many variations in the simplified process shown in the drawing are possible, as further described.

As is further pointed out below, several of the steps generally described above may be varied according to conditions, materials, number of copies desired, etc. This process is capable of producing replicas of exceedingly high quality from an original which is very sensitive to pressure, solvent, or heat damage.

The original surface deformation image to be replicated may be prepared by anysuitable method. The technique of this invention is especially suitable for replicating phase holograms of the sort described in parent application Ser, No. 521,982, filed Jan. 20, 196 6. This replicating technique is also useful for making replicas of frost images of the sort described in U.S. Pat. 3,196,001 and relief images of the sort described in US. Pat. 3,005,006. Surface deformation images formed by any of these techniques are generally fragile and susceptible to damage {by heat or abrasion. These images are generally formed by forming an electrostatic charge pattern on a surface of a deformable thermoplastic, and heating the thermoplastic to its softening temperature whereby the deformation image spontaneously forms. Thus, these images are generally formed on thermoplastics which soften only slightly above room temperature.

Any suitable deformable insulating thermoplastic layer may be used to form the original from which replicas are to be made. Typical insulating thermoplastics include the glycerol and pentaerythritol esters of partially hydrogenerated rosen, polyalpha methyl styrene, terpolymers of styrene, indene and isoprene; Piccolyte S- and S-100 (polyterpene resins made from beta pinene available from Pennsylvania Industrial Chemical Company and having ring and ball melting points of 70 and 100 C., respectively); Piccopale 70 SF and Piccopale (nonreactive olefindiene resins available from Pennsylvania Industrial Chemical Company, having melting points of 70 and 85 C. and molecular weights of 800 and 1,000 respectively); Piccolastic A-75, D-l00 and E-lOO (po1y styrene resins with melting points of 75, and 100 C., respectively available from Pennsylvania Industrial Chemical Company); Amberol ST137X (an unreactive, unmodified phenolformaldehyde resin available from the Rohm & Haas Chemical Company); Neolyn 23 (an alkyd resin available from Hercules Chemical Company); poly carbonates, polysulfones, poly(vinylchloride), mixtures of low molecular weight silicone and styrene resins, and mixtures thereof. It is preferred that the thermoplastic soften slightly above room temperature and be substantially insulating at the softening temperature so that the surface deforms properly in response to surface charge patterns. Generally, these preferred materials are slightly soft and tacky at room temperature and surface deformation patterns formed using them are susceptible to degradation upon slight heating or contact with most organic solvents. Therefore, care must be used in handling imaged sheets.

Where desired, the thermoplastic insulating layer may be coated over a photoconductive insulating layer or may be made photoconductive by incorporating suitable photoconductive materials therein or by sensitizin the resin to form a photoconductive charge transfer complex. Typical overcoatable photoconductive layers include amorphous selenium; pigment binder layers including photoconductive pigments such as cadmium sulfide, cadmium selenide, zinc sulfide, zinc selenide, zinc oxide, lead oxide, titanium dioxide, lead iodide, lead selenide, dispersed in an insulating film-forming binder such as a silicone resin, a styrene-butadiene resin or the like. Suitable organic photoconductors may also be formed into overcoatable layers or may be mixed into the heat deformable layer. Where desired, these photoconductors may be sensitized, such as with small amounts of dyes or Lewis acids, Typical organic photoconductors include 1,4-dicyano-naphthalene; 2,5-bis-(p-amino-phenyl)l,3,4-oxidiazole; N- vinyl carbazole; phthalocyanines, quinacridones, and mixtures thereof. Where the deformable layer comprises a suitable aromatic polymer, the layer itself may be made photoconductive by complexing it with a suitable Lewis acid. Typical Lewis acids include 2,4,7-trinitro-9-fiuorenone, 4,4-bis-(dimethyl-amino) benzophenone, tetrachlorophthalic anhydride, chloranil, picric acid, 1,3,5-trinitrobenzene, and mixtures thereof.

The surface deformation image may be formed on the heat deformable layer by any suitable technique. Broadly speaking the image forming steps include uniformly electrostatically charging the surface of the layer, exposing the surface to a light pattern to be reproduced and softening the layer to permit the surface deformation image to spontaneously form. Where the deformable layer and/ or photoconductive layer are supported on a conductive substrate such as metal foil plates or polymeric films having a transparent conductive coating, uniform electrostatic charge may be formed, for example, by corona discharge as described by Carlson in US. Pat. 2,588,699. Of course, the uniform electrostatic charging may be accomplished by other techniques, such as tri'boelectric friction charging as described by Carlson in U.S Pat. 2,297,- 691 if desired. Where the heat deformable layer and/or photoconductive layer are self-supporting or formed on an insulating substrate, the uniform char e may be formed, for example, by the double corona technique described by Gundlach in US. Pat. 2,885,556. Where a frost or relief image is to be formed, exposure will be to a pattern of light-and-shadow to be reproduced. Where the deformable layer is self-photoconductive, the exposure step may be immediately followed by development. Where the deformable layer is coated over a photoconductor, it is generally desirable to recharge the surface of the deformable layer, as by corona charging, to form a surface charge pattern suitable for development, Where it is desired to produce a phase hologram, exposure may be by the techniques described in the above-cited parent application. Development may be by any suitable technique which softens the surface of the deformable layer allowing the deformation image to form, In general, it is preferable to heat the plate to the softening temperature of the deformable layer, allow the deformation image to form and then cool below the softening temperature of the layer to fix the image. However, if desired, the surface may be softened by application of a solvent liquid or vapor to the surface of the deformable layer.

These techniques form ori inal images of high resolution on materials which are subject to degradation by heat, solvents or abrasion. The deformation images are generally erased if the surface of the deformation layer is softened by heat or solvent contact after the image forming charge pattern has dissipated.

As a first step in the formation of replicas from a surface deformation original, it is preferred that the original. be treated so as to form a surface skin having a thickness greater than about 0.3 micron. The fixing of surface deformation originals by forming such surface skins is further described in copending application Ser. No. 388,324, filed Aug. 7, 1964. This surface skin is harder and less solvent soluble, hence tends to make the original more resistant to erasure by abrasion, slight heating or contact with small amounts of solvent vapor. Any suitable method may be used to form the surface skin. This may be by in situ formation or by deposition of one material on a second material. Typical methods of skin formation include exposure to actinic light, X-rays, beta rays, gamma rays, electrical bombardment, corona discharge, high voltage discharge, exposure to visible light, exposure to air, contact with chemical means such as oxidizing agents and/or cross linking agents, the addition of sensitizers to increase the sensitivity of the heat-deform able layer to skin forming means, spraying, dip coating, and any suitable combination of these techniques. Depending upon the composition of the surface deformable layer, the degree and extent of exposure to these various radiation means may be varied to form a suitable surface skin. It has been found with the generally preferred der formable materials, exposure to ultraviolet radiation for a sufiicient period gives a surface skin having excellent fixing characteristics. Therefore, ultraviolet exposure is a preferred method of forming the surface skin.

The second step in the replication process of this inven tion is the preparation of a negative replica by casting against the original surface deformation image.

Any suitable hardenable material may be used to form the negative replica. It is important that the material be capable of producing a high resolution replica. Very loW shrinkage as the material solidifies is required. Also, the material must not require heating above about 200 F. to solidify nor may appreciable heat be generated during hardening. Furthermore, since most of the materials from which surface deformation originals are prepared are sus= ceptible to damage by most organic solvents the casting material should not include appreciable amounts of image degrading solvents. Typical hardenable materials include low melting metal alloys such as Cerralow 117, a bismuth indium alloy available from the Cerro Corporation; waxes such as Epolene Cl2, a low molecular weight polyeth ylene wax available from Eastman Chemical Co.; gelatin such as the food product produced by the Knox Gelatin Company, polyvinyl alcohols such as Elvanol 71-30 and Elvanol 72-60 available from Du Pont Electrochemical; one or multiple component silicone rubbers such as RTV- ll, RTV-20, RTV-60, RTV112, RTV-116 and RTV- 118 available from General Electric, Silastic RTV-501, Silastic RTV S-5137A, Silastic RTV S-5138A, Silastic RTV S-5302 and Silastic RTV S5303 available from Dow Corning; and mixturesthereof. It is preferred that this first negative replica be prepared using a curable material. Optimum results are obtained with silicone rubber compounds which cure at temperatures below about 200 F. Silicone rubbers provided the highest resolution replica of any hardenable material tested. Silicone rubbers may be formed from silicone gums. These gums are largely made up of polymers of dimethyl silicone. The gums may contain dimethyl siloxanes copolymerized with minor amounts of another difunctional silicone. For example, polydimethylsiloxanes may be copolymerized with about to about 15 percent of diphenyl silicone, diethyl silicone or methylphenyl silicone. The silicone gum may also contain active sites such as SiH, SiOH, or SiOC H and groups such as vinyl, fluorocarbon or nitrile groups may be introduced into the silicon-e molecule. If desired, finely divided fillers such as silica gel, calcium carbonate, titanium dioxide, iron oxide or mixtures thereof may be mixed with the silicone gum to increase the tensile strength of the ultimate silicone rubber replica. Generally, image resolution increases with a decrease in average filler particle diameter. Typical fillers range in diameter from about millimicrons to about one micron. Any suitable catalyst such as metal soaps, peroxides and other materials capable of generating free radicals may be used to cure silicone gum compositions which require the addition of a catalyst. Typical catalysts include benzoyl peroxide, dichlorobenzoyl peroxide, di-tert-butyl peroxide, t-butyl peroxide and mixtures thereof. The rate of curing depends upon the relative quantity of catalyst employed. Satisfactory curing rates are achieved when up to about 5 percent by weight of catalyst based on the weight of the silicone rubber is used. Curing is believed to occur by the formation of siloxane crosslinks between polymer chains. The curable material is placed in a vacuum to remove entrapped air and then introduced into a mold in contact with the surface deformation image to be reproduced. The casting material is allowed to cure after which it is peeled from the original.

The next step in the replication process of this invention is the preparaton of a second generation positive replica by casting from the first generation negative replica. Any suitable film-forming material which can be liquified and which does not attack the first generation replica may be used. The replication casting material may be prepared by dissolving a suitable film forming material in a solvent. Wide latitude in the relative quantity of solvent employed is permitted, the ultimate formation of a continuous film being the principal limiting factor. Typical film forming materials include solutions of polystyrene in toluene, polymethyl methacrylate in methyl ethyl ketone, polyethyl methacrylate in ethylene dichloride, cellulose acetate butyrate in ethyl acetate, curable epoxy resins, curable polyester resins, molten polystyrene, molten polymethyl methacrylate, molten polyethylene, plastisols such as polyvinyl chloride resin dispersed in Z-ethylphenylester, and mixtures thereof. Acrylic resins, particularly polyisobutyl methacrylate, are preferred because they harden uniformly without bubble formation and are compatible with the generally used substrate, which are often acrylic materials. Therefore, these are the preferred materials for the production of the second generation positive replica. In order to form a flat second generation plastic replica, the casting material is formed into a thin liquid layer on a supporting member and/or on the first generation replica. During, prior or subsequent to solidification of the liquid layer or layers, the supporting member is placed upon the first generation replica under slight pressure. If solidification has not occurred prior to assembly, the liquid layer or layers is allowed to cure or dry for a suitable time. The resulting casting is a rigid, fiat second generation (positive) replica of the original surface deformation image. If a liquid layer of casting material on the surface of the first generation replica is allowed to solidify prior to contact with the supporting member, transfer of the solidified layer to the supporting member is oc casionally enhanced 'by the application of heat to the supporting member prior to or during transfer, especially if the solidified layer is non-tacky. Substantial solidification of the liquid layer on the surface of the first generation replica prior to transfer is preferred because greater production rates are achieved, particularly when film forming materials dissolved in a fugitive solvent are employed. Where pressure transfer of the solidified material is not sufficient to cause the cast layer to adhere to the supporting member, solvent softening of the cast layer and/or supporting member or any suitable conventional adhesive may be employed to effect bonding of the cast layer to the supporting member. The support member may be of any convenient thickness, rigid or flexible, solvent soluble or insoluble, transparent or opaque, and may be in any desired form such as a web, sheet, plate or the like. The support member should, however, have at least one smooth surface which can support the second generation replica without unduly distorting the image. The first generation replica may be employed as a master to form a plurality of final second generation positive replicas. The preferred technique of forming the final second generation positive replica includes the steps of solidifying the hardenable casting material to a tacky state and transferring the solidified casting material with the aid of pressure to the surface of a support member. The pressure employed to effect transfer is not particularly critical, but should be sufficient to at least bring about good contact between the casting material and the surface of the support member. Excellent replicas are achieved when the casting material is suificiently tacky to stick or adhere (transfer), within five seconds, under slight pressure, e.g., thumb pressure, to a sheet of clear cellulose acetate and remain adhered thereto after separation of the first generation replica from the casting material. Although molten or curable casting materials may beemployed, optimum results are achieved with solutions of film forming polymers in volatile solvents. Solutions of film forming polymers are found to reach the tacky state in less time than other hardenable materials with' less expenditure of energy and to provide higher resolution images. The quantity of casting solution should be sufficient to form a dried layer having a smooth deformation free outer surface.

If a metal master is desired, a metal surfaced replica is prepared. After chemically cleaning and sensitizing the surface of the second generation replica, a metal such as silver is chemically reduced and sprayed upon the second generation replica to a thickness sufficient to insure the absence of pinholes and sufiicient to bear the subsequently employed electric plating current. This thickness is preferably about 3 or 4X10 inches for maximum avoidance of pinholes and warping during the electro-forming step. This coating provides the hghly conductive surface required to produce a good electroformed metal master replica. A typical suitable process for sensitizing and silvering is described by A. M. Max in Application of Electroforming to the Manufacturing of Disk Records," ASTM Special Technical Publication No. 318 (1962). The second generaton replica is then placed in a nickel sulfamate electroforming bath and nickel is electroplated onto the conductive surface. The plating bath is held at a temperature of about F. while the current drawn for a 12 inch disk is about 50 amperes. The electroforming is continued until the layer has suflicient thickness to be self-supporting. Optimum layer integrity and maximum freedom from image distortion is achieved with a nickel layer thickness between about 0.005 to about 0.012 inch. Then, the metallic layer is lifted from the plastic, from which it separates easily, to yield a third generation silvered metal master (negative) replica. This metal master may now be used to stamp replicas of the original surface deformation image by the stamping processes described below. However, if desired, a more durable metal master may be prepared. Nickel sulfamate electroforming baths are preferred because of the rapid and uniform deposition achieved.

To form a higher generation more durable metal replica, the surface of the third generation metal master negative replica is treated with a potassium dichromate solution to chemically passivate its surface. This treatment permits the electroforming of an all-nickel replica of the silvered surface but prevents a metal to metal bond from forming. This results in a durable, nickel (positive) fourth generation replica. If desired, a durable fifth generation nickel (negative) replica may be formed from the fourth generation replica. Also, if desired, the surface may be given a chromium flash to increase the life of the stamper.

Whichever technique is used to produce the final metal stamping master, the ultimate plural replicas of the original surface deformation image are prepared by the technique described above to form a plurality of final second generation replicas or by stamping. The following procedure is illustrative of the hot stamping technique of this invention. In general, a thin dry thermoplastic film (typically, cellulose acetate having a thickness of from about 0.003 to about 0.010 inch) is pressed between a heated nickel stamper and a press-pad which may be fiat or may have the overall contours of a positive master conforming to the contour of the negative stamper. The thermoplastic film is preheated to a suitable temperature so that there are no thermal shock effects when it is pressed against the stamper of even higher temperature. Since the depth of the surface deformation image, especially in the case of the holographic original, is very slight (on the order of half micron) there is little or no net fiow of plastic but rather only a slight displacement. At the end of the pressing time, the pressed thermoplastic is released. Surprisingly, release may be effected without any pro-release cooling. The result is a good quality, hot stamped replica of the original surface deformation image. Any suitable thermoplastic material may be used in this hot stamping process. Typical thermoplastics include acetates such as cellulose acetate, cellulose acetate butyrate, cellulose triacetate, butyrates, polycarbonates; polyesters such as polyethylene terephthalate; vinyl resins such as polyvinyl chloride, polysulfone resins, and mixtures thereof. Satisfactory images are obtained on most thermoplastic materials tested with stamping pressure between about 400 p.s.i.g. to about 6,000 p.s.i.g., a thermoplastic film preheat temperature up to about the softening range of the thermoplastic film material, a stamping master temperature from about 120 F. to about 400 F. and a pressing time less than about 30 seconds. Generally, higher pressures are necessary when lower stamping master temperatures are employed. It has been found that the highest quality replicas are produced on cellulose acetate with a stamping pressure of from about 400 to 6,000 p.s.i.g., a preheat temperature up to about 200 F., a stamping master temperature of from about 120 F. to about 300 F., and a pressing time of from about 1 to seconds. These, therefore, are preferred parameters for the hot pressing of the highest fidelity replicas on cellulose acetate. However, any other suitable material may be used to produce the final replica. Cold stamping may be substituted for the hot stamping technique described above. However, the hot stamping technique is preferred because less pressure is required and mold erosion is significantly reduced. Typically, metallic coated plastic films may be used to produce reflection type holographic replicas. Alternatively, a clear plastic replica may be metallized after the replica is formed. Both sides of the plastic sheet may be stamped simultaneously or sequentially with the same or different holograms. Of course, the stamping master may be either planar or formed on the surface of a cylinder suitable for rolling across the thermoplastic sheets.

It should be noted that with the above described hot stamping process, the thermoplastic film is held under pressure for comparatively short time and is then released hot leaving the softened plastic unconstrained by the mold during cooling. Surprisingly, distortion is not evident from direct viewing of the image despite the fact that the material was not cooled before removal from the mold. This obviates a mold temperature reduction step after the formation of each replica and permits extremely rapid production of a series of very high quality replicas from a single master. Unlike solvent softening i0 processes, this hot stamping process has the advantage of being completely dry and free of solvent recovery or disposal problems.

DESCRIPTION OF PREFERRED EMBODIMENTS The following examples further point out the advantages of the surface deformation image replication process of the present invention. All parts are by weight unless otherwise indicated. The following examples should be considered to represent preferred embodiments of the process of the present invention.

EXAMPLE I .layer of Staybelite Ester 10 (a glycerol ester of 60 percent hydrogenated rosin available from the Hercules Chemical Company) to a dry thickness of about 1 micron. This layer is applied by withdrawing the substrate from a 20 percent solution of the resin in a kerosene solvent at a rate of about 5 inches per minute. After drying, the com pleted imaging member consists of a conductive base, a photoconductive layer and a deformable thermoplastic layer. This plate is corona charged in darkness to a surface potential of about 500 volts. The plate is then exposed in a system of the sort shown in FIG. 1 of copending parent application Se'r. No. 521,982, filed Jan. 20, 1966. The exposure is to object and reference beams using a helium neon continuous wave laser operated in the term 00 mode at 6328 angstrom units (Model 5200 available from the Perkin Elmer Company). The reference beam is brought in at an angle of about 30 degrees.

After rechanging, the plate is then slowly heated to its softening temperature. The reconstructed image is viewed simultaneous with the formation of the hologram. After the deformations are well formed, the plate is cooled to fix the image. A holographic record of excellent quality with resolution of about 800 lines per millimeter is thus produced.

The hologram is exposed for about 6 hours to ultra violet light from a General Electric watt mercury arc lamp at a distance of approximately 6 inches. As described above, this toughens the surface and makes it less susceptible to damage.

About 100 parts RTV-1l (a room temperature vulcanizing silicone rubber composition available from the General Electric Company) is mixed with about 0.3 part of catalyst supplied with the rubber. This mixture is placed in a vacuum desiccator at a pressure of less than 2 p.s.i.g. to allow trapped air to leave the mixture. Froth produced by the exiting trapped air is observable on the surface of the mixture. After about 5 minutes, the pressure is slowly brought up to ambient pressure. The original hologram is then placed face up in the bottom of a rectangular, half-inch mold. After skimming off the upper surface of the evacuated silicone rubber mixture, the mix ture is carefully poured into the mold to a level of approximately /8 inch to insure that all of the contours will be completely filled. The resin is then allowed to cure at room temperature for about 48 hours. With larger amounts of catalyst, the cure time may be shortened considerably. The cured rubber is then peeled from the original. The result is a flexible, rubber, durable first generation negative master replica. The original hologram is undamaged by this casting process.

A castingsolution is then prepared by dissolving about 17 parts powdered Elvacite No. 2045 (a polyisobutyl methacrylate resin available from E. I. du Pont de Nemours & Company) and about 28 parts purified ethylene dichloride. The solution is allowed to stand for several hours to let air bubbles escape from the stirred liquid. A piece of clear Plexiglas (polymethyl methacrylate available from Rohm & Haas Company) having a thickness of about .4 inch is cut to size slightly larger than the rubber master. A thick stream of the replicating solution is poured along one edge of the Plexiglas substrateand along one edge of the rubber mold. Glass stirring rods are then used to roll the streams into a thin layer of solution over both the plastic and rubber surfaces. Immediately, the coated plastic substrate is placed upon the coated rubber in a manner which resembles a hinged door moving to closed position. Any air bubbles remaining will be squeezed out during this operation. Finally, a weight sufficient to produce about 0.18 p.s.i.a. on the solution interface is placed on the plastic substrate and the system is allowed to dry for approximately 3 hours at room temperature. The result is a rigid, transparent plastic second generation positive replica of high quality. The silicone rubber first generation master is undamaged by this casting process.

A metal hologram replica is then prepared. After chemically cleaning and sensitizing the surface of the second generation plastic replica with a stannous chloride solution, silver is chemically reduced from an ammoniacal silver nitrate solution while being sprayed on the plastic replica. The resulting silver layer has a thickness of approximately 3 l0 inches. The silver coating provides the highly conductive surface required to produce a good electroformed metal replica. The nickel layer having a thickness of about 0.012 inch is then electroformed onto the silver surface in a nickel sulfamate bath at a temperature of about 130 F. and a total current for a 12 inch disk of about 50 amperes. After electroforming, the nickel backed silver is easily lifted from the plastic to yield a third generation, negative, metal master replica with a silver surface. This master is suitable for use as a short run stamper.

A plurality of replicas of the original hologram are then produced using the metal master. A dry cellulose acetate film having a thickness of about 0.007 inch is preheated to about 190 F. The metal master is heated to about 280 F. and is pressed against the preheated cellulose acetate film at a pressure of about 800 p.s.i.g. against a flat press-pad. Pressure and temperature are maintained for about seconds. At the end of the pressing time, the pressed plastic film is released without any prerelease cooling. The result is an excellent quality replica of the original hologram. Over 100 additional replicas are similarly hot pressed before quality of the replica begins to noticeably decrease.

EXAMPLE II An original hologram, a catalytic cast first generation and a solvent cast second generation masters are prepared as in Example I. In this case, it is desired to prepare a more durable hot pressing master so that a greater number of replicas may be produced. After the plastic replica has been coated with silver and a 0.012 inch layer of nickel has been electroformed thereon, the silver surface is treated with a potassium dichromate solution to passivate the surface. This permits the electroforming of another nickel layer against the silver surface without producing silver-to-nickel bonding. After a 0.01 inch layer of nickel is thus electroformed, it is separated from the silvernickel master. The result is a durable, all-nickel (positive) fourth generation master. This may be used in a hot stamping process as described in Example I producing stamped negative plastic replicas of the original. For many holographic uses, the negative replica is suitable. If desired, this metal master may be given a chromium flash for about 1 minute drawing about 375 amperes to a 14 inch disk area to further increase the life of the stamper.

12 EXAMPLE III The all-nickel (positive) fourth generation master of Example II is knife coated with a solution of about 16 grams of polyisobutyl methacrylate dissolved in about 200 milliliters of methyl ethyl ketone. The resulting liquid film is permitted to dry in air at room temperature for about 20 seconds until it reaches a tacky state. A thin sheet of cellulose acetate having a thickness of about 5 mils is uniformly pressed into contact with the tacky film by means of a rubber hand roller. The cellulose acetate sheet is then peeled away from the fourth generation master. The transferred film exhibits good adhesion to the cellulose acetate sheet. Over additional replicas are similarly formed with the fourth generation master with no noticeable decrease in quality.

EXAMPLE IV An original hologram, and a catalytically cast first generation master are prepared as in Example I. In this case, it is desired to prepare a second generation replica at a more rapid rate than that illustrated in Example I. The first generation master is dipped in a bath of about 2 grams of polymethyl methacrylate dissolved in 200 milliliters of methyl ethyl ketone and smoothly withdrawn over a period of about 3 seconds. The resulting liquid film is permitted to dry in air at room temperature for about 30 seconds until it reaches a tacky state. The tacky film is then pressed into contact with a thin sheet of cellulose acetate butyrate having a thickness of about 10 mils. The first generation master is peeled away immediately after pressure contact with the cellulose acetate butyrate sheet is established. The transferred film exhibits surprisingly strong adhesion to the cellulose acetate sheet. The total time required to form the second generation replica is approximately 360 times faster than the second generation master making steps described in Example I. The first generation master is reused in the manner described above to produce over 100 good quality second generation replicas with no discernable decrease in quality. One of the second generation replicas thus formed is then used to make a third generation master as described in Example I.

EXAMPLE V The process described in Example IV is repeated except that a 12 mil sheet of polyethylene terephthalate is substituted for the cellulose acetate butyrate sheet. The second generation replicas formed are equal in quality to those obtained in Example IV.

EXAMPLE VI The process described in Example IV is repeated ex-= cept that the liquid film is roll coated onto the surface of the first generation master, allowed to dry for about one minute to a non-tacky state, and then pressed for about 4 seconds against a cellulose acetate butyrate sheet supported on a hot plate maintained at a temperature of about 100 C. The first generation master is then separated from the transferred film. The first generation master is reused in the manner described above to produce over 100 good quality second generation replicas. One of the resulting second generation replicas is then used to make a third generation master as described in Example I.

Although specific components and proportions have been stated in the above description of preferred embodiments of the materials utilized in the process of this invention, other materials and conditions as listed above, where suitable, may be used with similar results. In addition, other materials may be added to the various replicating materials, etc., to synergize, enhance, or otherwise modify their respective properties. For example, temperature variations or mold release agents may be used if desired.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of this disclosure. These are intended to be in cluded within the scope of this invention.

What is claimed is:

1. The method of preparing a plurality of substantially identical replicas of an original surface deformation image which comprises:

(a) forming a negative master of said original deforma tion image;

(b) placing a film of thermoplastic material having a softening temperature of from about 120 F. to about 400 F. on a backing surface;

() preheating said thermoplastic film to about the softening temperature;

((1) heating said master to a stamping temperature of,

from about 120 F. to about 400 F., provided the stamping temperature is above the softening temperature of the thermoplastic material;

(e) pressing said master against said thermoplastic material, for a period of up to about 30 seconds at a pressure of from about 400 to 6,000 p.s.i.g.;

(f) stripping the positive replica from the master while said master is maintained at about the stamping temperature; and

(g) repeating steps (b) through (f) at least one addi tional time with fresh thermoplastic .material.

2. The method of claim 1 wherein the amplitude difference between raised and depressed portions of said surface deformation image is less than about 1 micron.

3. The method of claim 1 wherein said thermoplastic material has a thickness of from about 0.003 to about 0.0l0inch.

4. Themethod of claim 1 wherein said master temperature is from about 120 F. to about 300 F. and the pressing time is from about 1 to about seconds.

5. The method of claim 1 wherein said thermoplastic film is cellulose acetate.

6. The method of claim 1 wherein said surface deformation image is a hologram.

7. The process of making a plurality of replicas of a surface deformation image which comprises the steps of:

(a) forming a first generation negative replica of the surface deformation image by forming against a de formation image bearing surface a layer of a harden able material which is inert to the image bearing sur face, hardening said layer and stripping the thus formed negative replica therefrom;

(b) forrning a second generation positive replica by casting a liquified resin against said first generation negative replica and permitting said liquified resin to at least partially solidify, removing the thus formed second generation positive replica from said first generation replica;

(c) forming a third generation negative replica by depositing a thin layer of silver on the surface of said second generation positive replica, electroforming a layer of nickel on said silver layer, and separating 14 said silver iayer from said second generation posi= tive replica to provide a negative metal master;

(d) preheating a film of thermoplastic material having a softening temperature of from about F. to about 400 F. to about the softening temperature;

(e) heating said master to a stamping temperature of from about 120 F. to about 400 E, provided the stamping temperature is above the softening tem perature of the thermoplastic material;

(f) pressing said heated metal master against said preheated thermoplastic material for a period up to about 30 seconds at a pressure of from about 400 to 6,000 p. s.i.g.;

(g) stripping the thermoplastic replica from said metal master while said metal master is maintained at about the stamping temperature; and

(h) repeating steps (d) through (g) at least one additional time with fresh thermoplastic material.

8. The method of claim 7 wherein the amplitude dit ference between raised and depressed portions of said surface deformation image is less than about 1 micron.

9. The method of claim 7 wherein said thermoplastic material has a thickness of from about 0.003 to about 0.010 inch.

10. The method of claim 7 wherein said master tem= perature is from about 120 F. to about 300 F. and the pressing time is from about 1 to about 10 seconds.

11. The method of claim 7 wherein said thermoplastic film is cellulose acetate.

12. The method of claim 7 wherein said surface de= formation image is a hologram.

13. The method of claim 7 wherein said hardenable material is a curable silicone rubber.

14. The method of claim 7 wherein said second gen eration positive replica is formed from a solution of an acrylic resin in an organic solvent.

References Cited UNITED STATES PATENTS 2,551,005 '5/1951 Johnson 264-284 3,311,692 3/1967 Baird a- 264-293 2,768,133 10/1956 Lundbye 204-20 2,854,337 9/1958 Pearson -1 264-1 2,875,543 3/1959 Sylvester et al. 264-1 3,055,006 9/1962 Dreyfoos et al. 3,265,776 8/1966 Henkes 264-4 3,364,090 1/ 1968 Slipp 264-1 FOREIGN PATENTS 3,222 1889 Great Britain 1. 204-3 DONALD J. ARNOLD, Primary Examiner A. H. KOECK-ERT, Assistant Examiner US. Cl. X.R. 

